I'm no expert, but I think my luci configuration seems to be broken. In my current file the section config internal themes is empty and I think it should contain at least option Bootstrap '/luci-static/bootstrap'. Without that, you cannot save the System / System configuration because there are no "Designs" /empty dropdown list) on the "Language and style" tab. So luci complains that Option "Design" contains an invalid input value..
I'm running OpenWrt 19.07.3 r11063-85e04e9f46 / LuCI openwrt-19.07 branch git-20.178.63701-ce35d08 and just did an update, containing the broken file. I'm not sure what to do about that, maybe some Dev can take care of that?
Usually themes register themselves in that file after they're installed. Looks as if you somehow managed to replace it with the basic version shipped by luci-base. You could either reinstall the bootstrap theme (overkill) or simply readd the option Bootstrap '/luci-static/bootstrap' line manually.
Not entirely sure how you got into this situation since opkg is supposed to preserve a modified /etc/config/luci when reinstalling luci-base. Did you copy the /etc/config/luci-opkg backup over the old /etc/config/luci ? If so, this would be the cause then.